Metal masks are formed by etching apertures or openings that correspond to a specific pattern on a metal sheet or foil. In a commonly used metal mask fabrication process, photolithography is used to form resist patterns on the metal workpiece on one or both sides of the metal piece, and apertures are formed by subtractive etching using the resist stencil(s) as a mask. The use of metal masks in electronics manufacturing is primarily for forming metallization features. In one use, the mask is placed adjacent to a substrate and the material of interest is evaporated, ion plated or electroplated through the openings in the mask. Another use is for screening paste, containing metallic particles, onto substrates. In the screening process, the metal mask is held against a substrate and a paste type material, which is usually finely ground metallic particles mixed with an organic binder, is squeezed through the openings of the metal mask. The metal mask is subsequently removed, thereby leaving a pattern of paste on the substrate corresponding to the metal mask pattern. The screened paste is heated to convert it into a conducting film.
Wet etching, using a resist stencil pattern, is the most widely used technique for forming metal masks. Other techniques such as electroforming, reactive etching and laser assisted etching have also been discussed as having some advantages for fabricating metal masks. In electronic applications, there is a continuous need to shrink the size of the minimum feature (aperture) that can be etched into the metal mask. This is true whether the application involves the use of evaporation or screening through the metal mask. In addition, a further requirement on the metal mask is the need for structural rigidity to withstand the handling in use so that the masks can be used several times. It is well known that metal masks made of very thin material are prone to bending and other forms of damage, which reduce their useful life. Therefore, most metal masks have a certain minimum thickness, depending on the application and size which is usually in thousands of an inch (mils). The widely used metals for metal mask fabrication are the following: molybdenum, tungsten, copper and nickel as individual layers or in a cladded form as a double layer. The choice has been due to their availability in preferred forms such as sheets or cladded layers and their amenability to electroforming and etching processes. Molybdenum, for example, is preferred for its structural rigidity and ease of obtaining in rolled sheet form with tightly controlled thickness.
In a technique using multilayer metal foils, Crimi, et al. (Crimi, J. S., et al., "Dimensionally Stable Metal Mask Fabrication Process," IBM Technical Disclosure Bulletin, Feb. 1976, pp. 3069-3070) describes a process in which a copper foil is plated with nickel on both sides followed by forming resist stencil patterns by known methods. In the Crimi, et al., process, the nickel layers are etched first using etchants selective to copper followed by etching copper using etchants selective to nickel. Thus, the Crimi, et al., process achieves a dimensionally stable process. Hodgson, et al. (Hodgson, R. T., et al., "Method for Laser Etching a Composite Mask," IBM Technical Disclosure Bulletin, Nov. 1988, pp. 5-6) discloses a process for laser etching of a composite mask, wherein the cladded Cu-Ni layers are etched in reactive gas species by irradiating with an excimer laser. This avoids the wet etching employed in the Crimi, et al., process, thereby avoiding undercuts in the mask etching profile. Etch profile refers to the etched contour of the aperture surface. This is discussed in more detail later. Another process by Kin, et al. (Kin, C. C., "RIE Made Molybdenum Mask," IBM Research Disclosure, May 1990, p. 313) achieves fine apertures in a metal mask by using reactive ion etching (RIE) of a molybdenum metal foil, coated with a RIE resistant layer, such as copper or nickel. In Kin, et al., process, pattern on the etch stop top layer was defined by wet etching, etching of molybdenum was carried out using CF4/02 or SF 6/02 gas plasma. Since the mask stock thickness is in hundreds of microns, the RIE process takes a long time in hours and is not easily manufacturable.
In the wet etch molybdenum metal mask process in order to achieve the smallest feature, the metal stock is usually etched from both sides by directional spraying to keep the isotropic (non-directional) etching component to a minimum. In wet etching, because of the isotropic nature of etching, the undercut with respect to a feature is of the same order of the thickness etched. To a first approximation, the etched diameter is equal to the resist feature diameter plus two times the depth of etching. The resist feature diameter for practical reasons has to be kept to some minimum value and not zero. The choice of etching from both directions keeps the thickness to etch roughly half the thickness of the metal stock. Thus to a first order approximation, the minimum feature size is equal to the thickness of the metal stock. The thickness of molybdenum sheet chosen for adequate handling strength has been 3 to 5 mils. Based on the above discussions, it can be seen that the realistically achievable limit of the mask aspect ratio in wet etching is roughly 1 (mask aspect ratio is defined as the ratio of the thickness of mask to minimum feature size).